Thermostatic switches, commonly referred to as thermal switches, are engineered for use in high reliability applications such as Space Science Satellites, Defense Satellites, Commercial Satellites, Manned Space Flight Programs and High-Value Terrestrial Applications. The operating and life specifications for thermal switches often require that the switches exhibit a high reliability while operating under extreme conditions such as within Space and Launch Vehicles. In addition, the thermal switches must often meet stringent temperature set point or threshold drift requirements over an operational life of typically twenty or more years.
The conventional thermal switches currently used for the above-identified applications may be bimetallic snap action type. A bimetallic disc is made of two dissimilar metals, where one metal has a low coefficient of thermal expansion and the other metal has a higher coefficient of thermal expansion. The bi-metal material is then punched into discs, formed, heat treated, and tested to meet desired temperature set point requirements.
The bimetallic disc deforms or actuates by changing from a convex state to a concave state at the desired temperature set point, which depends on the difference in thermal expansion coefficients of the two materials forming the bimetallic disc. Thus, the bimetallic disc alternates between a convex state and a concave state as the ambient temperature rises above or drops below the desired temperature set point.
At the set point temperature, the bimetallic disc moves either into or out of contact with a striker pin coupled to an armature, which may be a spring, such as a leaf spring. Depending on the design of the thermal switch, the deformation of the bimetallic disc causes the opening (e.g., open circuit) or closing (e.g., closed circuit) of a pair of electrical contacts or terminals. One example of a striker pin is described in U.S. Patent Publication No. 2004/0263311 (Thermal Switch Striker Pin) and is incorporated herein by reference in its entirety.
The components of the switch, such as the bimetallic disc, the striker pin, the armature, and portions of the terminals are located in a housing or case. The bimetallic disc is positioned between the striker pin and an internal surface of the case. Specifically, the amount of space or offset between the striker pin and the internal surface of the case is closely defined. By way of example, when the bimetallic disc is in the convex state it is in contact under force with the internal surface of the case due to its contact with the striker pin and when in the concave state it is in a free state under little or no force, yet remains in contact with the case.
Consequently, repeated actuation of the bimetallic disc has been known to cause an undesirable amount of wear to the disc, the striker pin, the case, or some combination of each. The amount of wear may become undesirable if it is sufficient to cause the set point temperature to “drift.” For example, the amount of wear may be undesirable if it causes a significant change in temperature in either the opening or the closing of the electrical circuit.